We propose to change the current approach to understanding the molecular basis of memory. Our approach challenges the current focus on quantitative identification of synaptic RNAs by developing new technologies to address protein synthesis-dependent synaptic plasticity: single- synapse-CLIP and synaptic translational profiling. These will allow us to redefine the problem from two new superimposed perspectives: the need to identify regulated RNA-protein complexes in specific synapses, and the need to define their role in translational regulation. Specific synapses will be studied: the apical dendrites of cerebellar Purkinje neurons (a site of motor learning), of CA1 pyramidal neurons in the stratum moleculare of the hippocampus (a site of associative memory), and of layer V pyramidal neurons of the visual cortex (a site of activity- dependent plasticity). Key RNA-protein complexes to be studied in these synapses will be Argonaute (Ago)-mRNA-miRNA ternary complexes, translationally regulated FMRP-mRNA complexes, and neuron-specific RNA regulatory protein-mRNA complexes known to be present in the dendrite and to bind 3' UTRs (nElavl (Hu proteins), Nova). These complexes will be compared with a delineation of all ribosome-mRNA synaptic complexes present in the same dendrites, allowing us to validate interactions by identifying translationally regulated synaptic mRNAs (synaptic translational profiling). Regulated dendritic RNAs will be further validated by assessing for their translational state in two well-studied paradigms of protein synthesis- dependent synaptic plasticity: that in the hippocampus, and in the visual cortex after dark rearng and subsequent light exposure. These studies will revolutionize our understanding of the nature and regulation of local synaptic mRNAs that underlie memory, setting the stage for new insight into neurologic diseases of memory such as Alzheimer's and other neurodegenerative diseases.